extensionsService: Really fix copying technical details

gdk_clipboard_set_text() is not introspectable, we have to use
gdk_clipboard_set_value() (shadowed as gdk_clipboard_set())
instead.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1796>

js/dbusServices/extensions/extensionsService.js

@@ -253,7 +253,7 @@ var ExtensionPrefsDialog = GObject.registerClass({
         });
         action.connect('activate', () => {
             const clipboard = this.get_display().get_clipboard();
-            clipboard.set_text(this._errorMarkdown);
+            clipboard.set(this._errorMarkdown);
         });
         this._actionGroup.add_action(action);